1. Field of the Invention
This invention relates to a medical device, and more particularly, to a novel clip to be used in the treatment of scoliosis.
2. Description of the Prior Art
Scoliosis is an abnormal lateral curvature of the spine which affects both young children as well as adults. There are many different types of scoliosis, but the common type seen is on a hereditary basis and is divided into very early mobile flexible curves without structural bony change and those which have fixed organic deformities of a structural nature within the vertebral bodies and adjacent soft tissue and bone structures.
The treatment of scoliosis is dependant on the severity of the curvature within the spine.
Minor curves require external bracing during the growing period in a child's life, but if the curve becomes more severe either in childhood or in adult life, major surgical straightening and correction is required to prevent otherwise severe progressive deformities of the spine, chest wall, and other organs within the chest and abdomen.
For a period of approximately 25 years the major method of treating severe spine curvatures surgically is the use of an implantable stainless steel rod which is placed adjacent to vertebral bodies and is hooked into the vertebral bodies in the portions on the posterior aspect called "laminae". Two hooks are placed at either end of the rod and the proximal or upper portion of the rod is ratcheted over a 2 inch portion of the length of the rod. This allows the hook with a small shoulder or ridge on the inside of the surface to fit onto the rod. This construction allows the hook to be moved along the rod in a series of steps thus distracting the two ends of the curvature through the vertebral bodies. Downward movement of the hook is prevented along the rods since the ridge on the interior portion of the opening of the hook interacts with the ratchet on the rod and the hook is displaced from its perpendicular orientation to the axis of the rod. If the hook is normal to the longitudinal axis of the rod it slides easily over the ratchet. However, if it is displaced from there, the ridge will be retained on the surface of each ratchet by preventing movement. Distraction forces are then applied to the vertebrae to correct and stabilize their orientation based on the ratchet principle on the concave side of the spine. This system is utilized not only for scoliosis but also to stabilize an otherwise fractured spine where two rods are used on either side of the spine and usually one or two vertebral bodies above the fractured level.
Although the Harrington rod system provides a very effective means of straightening the curved spine it unfortunately cannot solely maintain this position.
If a patient were to stand up shortly after surgery the weakest portion of the system is bone at the upper end of the rod termed the lamina, usually in the thoracic area. In this case when the patient became erect and the spine was loaded, the top hook would pull out of the bone taking a portion of the lamina with it and the system would then collapse and fail.
Because the spine in scoliosis is curved in a very complex, three dimensional manner, it is not possible to spread the load out in a routine fashion over more than one lamina as it is impossible to put on more than one hook on the ratchet system on a straight rod where the depth and rotation of each individual vertebra vary so considerably and also varies during the entire straightening process and thus only one hook can be used to take all the load in the proximal portion of the spine.
Once this straightening is obtained, in order to maintain the correction, bone fusion chips are taken from the hip area and the entire spine between the two hooks at either end of the correcting system is then covered with bone chips and bone graft which will eventually grow to incorporate the rods and the fusion mass into the entire area of the spine which has been straightened, corrected and fused.
A period of several months however is required until the fusion process is complete. During this time the spine must not be unduly loaded and thus external protection is required.
In the initial post-operative phase of anywhere from two to four weeks, the patient is kept recumbent on a special rigid frame termed the stryker bed.
At the end of this time, the patient is then placed in a full body cast and is allowed to be up walking. This cast prevents flexion, extension and rotatary forces being applied to the upper distraction hook at its insertion site. It takes anywhere from six to nine months before the fusion is solid and during this period of time the external supporting body cast is worn. The greatest disadvantage of the Harrington system is that all the load has been taken on one vertebral level at the upper end which is the weakest point of the system and more hooks could not be applied because the hooks lacked the ability to adjust and compensate for the different and constantly changing spine during the corrective procedure. If this were feasible the load could be distributed over a much greater area of the spine and yet allow fusion to occur while the patient could be up walking without external support.